Editing JAR Files

Anyone know how to edit JAR files? I downloaded a file for our firewall which enables RDP plugin through the vpn portal. I need to edit this jar file so I can change the resolution on it. I tried using a program called eclipse but could not understand that. Anyone have an easier way?

 

More info on this. The jar file when extracted has about 8 class files and 1 mf file. I'm assuming I have to edit one of those class files but not sure.

 

Those class files in the jar are compiled Java files.They can not be edited.
You will either have to try and get the actual source code, or;
Google 'Java decompiler' and try decompiling the class files you have. Then edit the decompiled files and re-build.